std::empty
Defined in header <array> |
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Defined in header <deque> |
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Defined in header <forward_list> |
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Defined in header <iterator> |
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Defined in header <list> |
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Defined in header <map> |
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Defined in header <regex> |
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Defined in header <set> |
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Defined in header <span> |
(since C++20) | |
Defined in header <string> |
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Defined in header <string_view> |
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Defined in header <unordered_map> |
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Defined in header <unordered_set> |
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Defined in header <vector> |
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(1) | ||
template <class C> constexpr auto empty(const C& c) -> decltype(c.empty()); |
(since C++17) (until C++20) |
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template <class C> [[nodiscard]] constexpr auto empty(const C& c) -> decltype(c.empty()); |
(since C++20) | |
(2) | ||
template <class T, std::size_t N> constexpr bool empty(const T (&array)[N]) noexcept; |
(since C++17) (until C++20) |
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template <class T, std::size_t N> [[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept; |
(since C++20) | |
(3) | ||
template <class E> constexpr bool empty(std::initializer_list<E> il) noexcept; |
(since C++17) (until C++20) |
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template <class E> [[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept; |
(since C++20) |
Returns whether the given range is empty.
1) returns
c.empty()
2) returns
false
3) returns
il.size() == 0
Parameters
c | - | a container or view with an empty member function |
array | - | an array of arbitrary type |
il | - | an initializer list |
Return value
true
if the range doesn't have any element.
Exceptions
1) May throw implementation-defined exceptions.
Notes
The overload for std::initializer_list
is necessary because it does not have a member function empty
.
Feature testing macro: __cpp_lib_nonmember_container_access
.
Possible implementation
First version |
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template <class C> [[nodiscard]] constexpr auto empty(const C& c) -> decltype(c.empty()) { return c.empty(); } |
Second version |
template <class T, std::size_t N> [[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept { return false; } |
Third version |
template <class E> [[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept { return il.size() == 0; } |
Example
#include <iostream> #include <vector> template <class T> void print(const T& container) { if ( std::empty(container) ) { std::cout << "Empty\n"; } else { std::cout << "Elements:"; for ( const auto& element : container ) std::cout << ' ' << element; std::cout << '\n'; } } int main() { std::vector<int> c = { 1, 2, 3 }; print(c); c.clear(); print(c); int array[] = { 4, 5, 6 }; print(array); auto il = { 7, 8, 9 }; print(il); }
Output:
Elements: 1 2 3 Empty Elements: 4 5 6 Elements: 7 8 9
See also
(C++20)
|
checks whether a range is empty (customization point object) |
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